The aromatic polycarbonate has excellent transparency, mechanical properties, thermal properties, electrical properties, weatherability, and the like and, by taking advantage of these characteristics, it is used as molded articles for optical use such as a light guide plate, a lens, an optical fiber, and the like. However, the light transmittance, one of the indicators of transparency thereof, is lower in comparison with polymethyl methacrylate (PMMA) and the like. Therefore, when a surface light source device is constituted from a light guide plate made of an aromatic polycarbonate and a light source, there has been a problem that luminance thereof is low.
Accordingly, there have heretofore been proposed several methods to enhance luminance of a light guide plate made of an aromatic polycarbonate. For example, Patent Literature 1 proposes a method of using a fluorescent bleaching agent and a bead-like cross-linked acrylic resin together, where the fluorescent bleaching agent improves the luminance and the bead-like cross-linked acrylic resin reduces nonuniformity in the luminance; Patent Literature 2 proposes a method of improving light transmittance and the luminance by the addition of an acrylic resin and an alicyclic epoxy; Patent Literature 3 proposes a method of improving the luminance by increasing efficiency of transcription of a concavo-convex pattern on the light guide plate by modifying the molecular terminal of the aromatic polycarbonate; Patent Literature 4 proposes a method of improving the luminance by introducing a copolyester carbonate having an aliphatic segment to increase efficiency of transcription of a concavo-convex pattern on the light guide plate; and the like.
However, in the method of Patent Literature 1, even though the luminance is partially improved, the light transmittance is reduced by the bead-like cross-linked acrylic resin and the fluorescent bleaching agent, resulting in significant reduction of the luminance in portions far from a light source of the light guide plate and making it difficult to obtain uniform luminance. In the method of Patent Literature 2, hue is improved by the addition of the acrylic resin but the light transmittance and the luminance cannot be improved because the aromatic polycarbonate resin becomes clouded; there is a possibility that transmittance is improved by the addition of the alicyclic epoxy but no improvement in the hue is observed. In the case of methods of Patent Literature 3 and Patent Literature 4, improvement effects in fluidity and transcription efficiency can be expected but there is a drawback that heat resistance deteriorates.
On the other hand, in the method of Patent Literature 5, the light transmittance and the luminance are improved by the addition of a polyoxyalkylene glycol or a fatty acid ester thereof, the glycol comprising mainly polyethylene glycol or polypropylene glycol. However, because heat resistance of the polyoxyalkylene glycol is low, molding at a temperature exceeding 320° C. or a long molding cycle causes severe yellowing and significant deterioration of the luminance and the light transmittance, thus causing a fear of adverse effects on the optical performance of the light guiding products. Furthermore, when molding is carried out at a temperature exceeding 340° C., decomposition gas of the polyoxyalkylene glycol causes occurrence of silver streaks on the surface of the molded article, making it impossible to fulfill its function as a light guiding product. For this reason, there is a restriction against raising temperature in order to improve fluidity, making it difficult to mold a thin and large-area light guide plate. Thus, use of the conventional aromatic polycarbonate has been limited to a molding compound for some small-sized light guide plate, which is molded at a low temperature around 280° C. Thus, the range of practical use of the aromatic polycarbonate has been narrow and insufficient.
Further, the methods disclosed in Patent Literature 6 and Patent Literature 7 comprise the addition of phenolic antioxidants and fluorescent whitening agents to the resin composition of Patent Literature 5, which are effective in decreasing occurrence of silver streaks but are ineffective in improving the yellowing problem. In this respect, the temperature at which molding can be performed is not much different from that in Patent Literature 5.